Fiber-reinforced organic resin matrix composites have a high strength-to-weight ratio, a high stiffness-to-weight radio and desirable fatigue characteristics that make them increasingly popular in aerospace applications. Therefore, composite materials are increasingly being used in the fabrication of structural components for aircraft.
A variety of techniques are used to join composite structures in aerospace and other applications. These fastening techniques include mechanical fastening, adhesive bonding and thermoplastic welding. Thermoplastic welding has numerous advantages over the other fastening techniques including the ability to join thermoplastic composite components at high speeds and with minimum touch labor and little, if any, pretreatments. The welding interlayer (which includes a susceptor and surrounding thermoplastic resin either coating or sandwiching the susceptor) also can simultaneously take the place of shims which are required in mechanical fastening. Therefore, composite welding promises to be an affordable fastening technique.
In the thermoplastic welding of thermoplastic and thermoset composite parts, the susceptor between the composite parts is heated and, in turn, heats and melts the resin of the parts. The melted resin functions as a hot melt adhesive at the welding interlayer between the parts. Upon subsequent cooling, the resin solidifies and secures the composite parts to each other.
In thermoplastic welding, it is desirable to heat the welding interlayer between the composite parts as uniformly as possible. Thermal uniformity and repeatability, as well as the amount of time necessary to develop acceptable tooling and parameters to meet these acceptable thermal uniformity conditions, has been a primary impediment to utilizing induction welding of thermoplastic composites. Extensive experimentation in developing the parameters has been used in applications in which induction parameters and tooling with heat sinks have been used. Often, however, the thermoplastic welding process is not selected due to these thermal uniformity issues.
Therefore, a thermoplastic welding apparatus and method in which an induced magnetic field is oriented parallel to the plane of the composite parts being welded and concentrates uniform heating at the joint between the composite parts, preventing or minimizing heating of the parts, is needed.